Important biological functions including maintenance of homeostasis in vivo, reproduction, development of individuals, metabolism, growth, control of the nervous, circulatory, immune, digestive or metabolic system, sensory adaptation, etc. are regulated by cells that receive endogenous factors such as various hormones and neurotransmitters or sensory stimulation like light or odor, via specific receptors present on cell membranes reserved for these factors or stimulation and interact with them. Many of these receptors for hormones or neurotransmitters by such functional regulation are coupled to guanine nucleotide-binding proteins (hereinafter, sometimes merely referred to as G proteins), and are characterized by developing a variety of functions through mediation of intracellular signal transduction via activation of the G proteins. In addition, these receptor proteins possess common seven transmembrane domains. Based on the foregoing, these receptors are thus collectively referred to as G protein-coupled receptors or seven transmembrane receptors. As such it is known that various hormones or neurotransmitters and their receptor proteins are present and interact with each other to play important roles for regulating the biological functions. However, it often remains unclear if there are any other unknown substances (hormones, neurotransmitters, etc.) and receptors to these substances.
In recent years, accumulated sequence information of human genome DNA or various human tissue-derived cDNA by random sequencing and rapid progress in gene analysis technology have been accelerating the investigation of human genome. Based on this, it has been clarified that there are many genes supposed to encode proteins with unknown functions. G protein-coupled receptors not only have seven transmembrane domains but many common sequences are present their nucleic acids or amino acids. Thus, they can be clearly identified to be G protein-coupled receptors in such proteins. On the other hand, these G protein-coupled receptor genes are obtained by polymerase chain reaction (hereinafter abbreviated as PCR) utilizing such a structural similarity. In these G protein-coupled receptors thus obtained so far, ligands to some receptors that are subtypes having high homology in structure to known receptors may be readily predictable but in most cases, their endogenous ligands are unpredictable so that ligands corresponding to these receptors are hardly found. For this reason, these receptors are termed orphan receptors. It is likely that unidentified endogenous ligands to such orphan receptors would take part in biological phenomena poorly analyzed because the ligands were unknown. When such ligands are associated with important physiological effects or pathologic conditions, it is expected that development of these receptor agonists or antagonists will result in breakthrough new drugs (Stadel, J. et al., TiPS, 18, 430-437, 1997; Marchese, A. et al., TiPS, 20, 370-375, 1999; Civelli, O. et al., Brain Res., 848, 63-65, 1999). Until now, however, there are few examples to actually identify ligands to orphan G protein-coupled receptors.
Recently, some groups attempted to investigate ligands to these orphan receptors and reported isolation/structural determination of ligands which are novel physiologically active peptides. Independently Reinsheid et al. and Meunier et al. introduced cDNA encoding orphan G protein-coupled receptor LC132 or ORL1 into animal cells to express a receptor, isolated a novel peptide from swine brain or rat brain extract, which was named orphanin FQ or nociceptin with reference to its response, and determined its sequence (Reinsheid, R. K. et al., Science, 270, 792-794, 1995; Meunier, J.-C. et al., Nature, 377, 532-535, 1995). This peptide was reported to be associated with pain. Further research on the receptor in knockout mouse reveals that the peptide takes part in memory (Manabe, T. et al., Nature, 394, 577-581, 1998).
Subsequently, novel peptides such as PrRP (prolactin releasing peptide), orexin, apelin, ghrelin and GALP (galanin-like peptide), etc. were isolated as ligands to orphan G protein-coupled receptors (Hinuma, S. et al., Nature, 393, 272-276, 1998; Sakurai, T. et al., Cell, 92, 573-585, 1998; Tatemoto, K. et al., Biohem. Biophys. Res. Commun., 251, 471-476, 1998; Kojima, M. et al., Nature, 402, 656-660, 1999; Ohtaki, T. et al., J. Biol. Chem., 274, 37041-37045, 1999). On the other hand, some receptors to physiologically active peptides, which were hitherto unknown, were clarified. It was revealed that a receptor to motilin associated with contraction of intestinal tracts was GPR38 (Feighner, S. D. et al., Science, 284, 2184-2188, 1999). Furthermore, SLC-1 was identified to be a receptor to MCH (Chambers, J. et al., Nature, 400, 261-265, 1999; Saito, Y. et al., Nature, 400, 265-269, 1999; Shimomura, Y. et al., Biochem. Biophys. Res. Commun., 261, 622-626, 1999; Lembo, P. M. C. et al., Nature Cell Biol., 1, 267-271, 1999; Bachner, D. et al., FEBS Lett., 457, 522-524, 1999). Also, GPR14 (SENR) was reported to be a receptor to urotensin II (Ames, R. S. et al., Nature, 401, 282-286, 1999; Mori, M. et al., Biochem. Biophys. Res. Commun., 265, 123-129, 1999; Nothacker, H. P. et al., Nature Cell Biol., 1, 383-385, 1999, Liu, Q. et al., Biochem. Biophys. Res. Commun., 266, 174-178, 1999). It was shown that MCH took part in obesity since its knockout mice showed the reduced body weight and lean phenotype (Shimada, M. et al., Nature, 396, 670-674, 1998), and because its receptor was revealed, it became possible to explore a receptor antagonist likely to be an antiobesity agent. It is further reported that urotensin II shows a potent action on the cardiocirculatory system, since it induces heart ischemia by intravenous injection to monkey (Ames, R. S. et al., Nature, 401, 282-286, 1999).
As described above, orphan receptors and ligands thereto often take part in a new physiological activity, and it is expected that their clarification will lead to development of new drugs. However, it is known that research on ligands to orphan receptors is accompanied by many difficulties. For example, it is generally unknown what secondary signal transduction system will take place after orphan receptors expressed on cells responded to ligands, and various response system should be examined. Moreover, tissues where ligands are present are not readily predictable so that various tissue extracts should be prepared. Furthermore, since an amount of ligand required to stimulate its receptor is sufficient even in an extremely low concentration when the ligand is a peptide, the amount of such a ligand present in vivo is a trace amount in many cases. In addition, a peptide is digested by peptidase to lose its activity, or undergoes non-specific adsorption so that its recovery becomes poor during purification. Thus, it is normally extremely difficult to extract such a ligand from the living body and isolate an amount of the ligand necessary for determination of its structure. The presence of many orphan receptors was unraveled, but only a very small part of ligands to these receptors were discovered so far due to the foregoing problems.